This invention relates to nanophotonic devices, and, more particularly, to optical resonator devices used in demultiplexing devices.
Wave-division multiplexing (WDM), and similarly, dense WDM (DWDM) and ultra-dense WDM (UDWDM), provide the ability to simultaneously transmit multiple signals through a single optical fiber or waveguide, with each signal being transmitted on a separate wavelength or channel and each typically carrying either 2.5 or 10-gigabit-per-second signals.
The International Telecommunications Union (ITU) has set standards for the basic wavelength and channel spacing used in WDM. Light, like radio waves has a wavelength. For light this is measured in nanometers (millionths of a millimeter). The ITU standards set a xe2x80x9cwindowxe2x80x9d from 1500 nm to 1535 nm for WDM, subdivided into 43 xe2x80x9cchannelsxe2x80x9d, sometimes referred to as xe2x80x9ccolorsxe2x80x9d, whose centers are separated by 0.8 nm. This represents a channel bandwidth of about 100 GHz regarded as the current practical limit for manufacturing precision tunable optical transceivers. In future, however, the channel spacing will be halved to provide up to 80 channels per fiber.
In practice, each channel can be treated as an independent optical transmission path and therefore can be modulated at whatever speed is appropriate for an application. A hierarchy of optical fiber transmission speeds has been standardized for the two major optical network systemsxe2x80x94Synchronous Optical NETwork (SONET) in the US, and the ITU""s standard Synchronous Digital Hierarchy (SDH) in the rest of the world. There are differences between the terminology and the details of hierarchy of speeds but the standards are not completely incompatible.
DWDM combines multiple optical signals so that they can be amplified as a group and transported over a single fiber or waveguide to increase capacity. Each signal carried can be at a different rate (OC-3/12/24, etc.) and in a different format (SONET, ATM, data, etc.) For example, a DWDM network with a mix of SONET signals operating at OC-48 (2.5 Gbps) and OC-192 (10 Gbps) over a DWDM infrastructure can achieve capacities of over 40 Gbps. A system with DWDM can achieve all this gracefully while maintaining the same degree of system performance, reliability, and robustness as current transport systemsxe2x80x94or even surpassing it. Future DWDM terminals will carry up to 80 wavelengths of OC-48, a total of 200 Gbps, or up to 40 wavelengths of OC-192, a total of 400 Gbpsxe2x80x94which is enough capacity to transmit 90,000 volumes of an encyclopedia in one second.
Micro-ring resonators are known in the prior art, such as that disclosed in U.S. Pat. No. 5,926,496. In addition, it is known in the prior art to use micro-ring resonators as filters, wherein the resonators act to separate desired wavelengths (i.e., channels) of a light signal from a DWDM input light signal. For example, FIG. 1 depicts a prior art filter arrangement 1 having an input waveguide 2, with an input port 3 and an output port 4, and an output waveguide 5, with an output port 6. A micro-ring resonator 7 is interposed between the input waveguide 2 and the output waveguide 5 and is tuned to a predetermined wavelength. To understand the operation of the filter 1, with a DWDM light signal propagating through the input waveguide 2 (in a direction from the input port 3 and towards the output port 4), part of the light signal (i.e., the wavelength of the input signal that is on-resonance with the resonator 7) will couple from the input waveguide to the resonator 7. That wavelength is thus demultiplexed or dropped from the input signal. The resonator 7, in turn, couples that wavelength to the output waveguide 5 and a light signal having that particular wavelength propagate through the output waveguide 5 towards the output port 6. The remaining wavelengths of the input signal, i.e., those which are not on-resonance with the resonator 7, by-pass the resonator 7 and continue propagating through the input waveguide 2 and towards the output port 4.
Using this basic methodology, full-scale demultiplexing systems have been built for lightwave systems. With reference to FIG. 2, a demultiplexing device 10 is shown having a single input waveguide 11, with an input port 12 and an output port 13. A series of micro-ring resonators 14A-D are arranged along the length of the input waveguide 11. Although not shown in FIG. 2, the resonators 14A-D would generally be each formed with a different radius; with the radius of the resonator determining, at least in part, the resonant wavelength of the resonator. Additionally, an output waveguide 15A-D is provided for each resonator 14A-D, with each output waveguide 15A-D having an output port 16A-D. The demultiplexing device 10 is referred to as a 1xc3x975 device: the first number (1) signifying a single input, while the second number signifies the number of outputs (5). Other combinations are possible, including 1xc3x978 and 1xc3x9716. With the structural arrangement of the device 10, a DWDM light signal propagating through the input waveguide 11, in a direction from the input port 12 and towards the output port 13, will be sequentially demultiplexed (also known as xe2x80x9cdemuxedxe2x80x9d) by the resonators 14A-D into four different wavelengths, with a remainder signal portion (i.e., those wavelengths that are not demuxed) propagating through the input waveguide 11. The various wavelengths will respectively propagate towards the output ports 13 and 16A-D.
With reference to FIG. 3, a chart is provided to symbolically represent the coupling of wavelengths of a light signal by a resonator. The arrows along line Axe2x80x2 represent different light signal wavelengths or channels LS. Trapezoidal blocks T on line Bxe2x80x2 represents the transfer characteristic of a resonator, such as resonator 7 (FIG. 3). With the DWM light signal having a plurality of wavelengths or channels propagating through input waveguide 2, the wavelengths or channels LS that coincide with the trapezoidal blocks T are coupled to the resonator 7, as represented by coupled wavelengths or channels CLS shown on line Cxe2x80x2 in FIG. 3. Wavelengths that are not coincident with the trapezoidal blocks T by-pass the resonator 7 and continue to propagate through (or are guided by) the input waveguide 2, as depicted on line Dxe2x80x2 and identified as SLS.
The spacing S between the trapezoidal blocks T is a free spectral range (FSR) characteristic of the resonator 7, whereas, the full-width half-maximum (FWHM) width W of the trapezoidal blocks T is indicative of the linewidth of the resonator 7. In addition, the finesse F of a resonator is equal to the FSR/linewidth. As can be appreciated, a narrow linewidth will result in a large finesse F, while a large linewidth will result in a small finesse F.
Although effective, the system of FIG. 2 has limitations. Each of the resonators 14A-D requires a narrow linewidth to only select a specific wavelength of the input signal. Where a large number of wavelengths are required to be demultiplexed, the finesse of the resonators 14A-D will be relatively high, thereby requiring relatively stringent tolerances, finer tunability, etc., and high manufacturing standards.
Thus, there exists a need in the art for an optical device that overcomes the above-described shortcomings of the prior art.
The subject invention overcomes the deficiencies of the prior art, wherein a demultiplexing device is provided for selectively demultiplexing wavelengths or channels of a DWDM light signal. The device includes a plurality of resonators, preferably micro-ring, which are arranged to xe2x80x9cslicexe2x80x9d a signal into wavelengths or channels (those terms being used interchangeably herein), rather than couple desired wavelengths. By xe2x80x9cslicingxe2x80x9d the signal in sequential steps, the resonators can each be formed with a lower finesse than resonators arranged in a prior art device. Prior art demultiplexing devices using resonators typically include a plurality of resonators arranged in a generally linear and cascaded array. All the resonators are required to have a finesse that is proportional to the number of wavelengths in the DWDM signal. For very broadband DWDM signals (high channel count), the required FSR is also proportionally larger, which means that the resonators will be very small.
With the subject invention, a plurality of resonators having different physical and optical characteristics are optically coupled to a plurality of waveguides, thus defining a plurality of stages. In each stage, the number of channels in the DWDM signal is reduced by two (or by 2N, an even number). Thus, the architecture of the present invention may also be referred to as a divide-by-2N architecture. After the first resonator stage, the DWDM signals will have been separated into even-number and odd number channels, with half of the channels being dropped by the first resonator. The other half will continue through the input waveguide, having by-passed the resonator.
Thereafter (i.e., in subsequent, downstream stages), a plurality of resonators are utilized to continue slicing, until single wavelength or channel light signals remain. It should be noted that the same number of resonators will generally be required for the subject invention as in a prior art demultiplexer. However, the resonators that are required can be of a much lower finesse than that of the resonators used in the prior art. In addition, the resonators will generally be arranged in parallel, thereby, cutting down the distance signals must propagate.
The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts which will be exemplified in the disclosure herein, and the scope of the invention will be indicated in the claims.